When detecting earth leakage currents in a single- or three-phase electrical circuit, a sensitive current transformer is needed to sense accurately current from as low as 30 mA or lower to as high as 3.0 A. A high-permeability core is used to minimize the error contribution of the magnetic core. Existing ways of optimizing the sensing accuracy of the current transformer include using a high-permeability core and shaping the core like a toroid. Another problem that occurs in optimizing sensing accuracy is known as load shift, which is an error that causes the output of the current transformer to shift due to load current. The current transformer senses a different current with and without a load current, and this difference is combined with an actual ground-fault signal and can add or subtract from the true ground-fault signal. This can cause the circuit breaker in which the current transformer is located to trip too early or too late (usually the latter). Accordingly, the ground-fault detection circuitry to which the current transformer is coupled needs to compensate for the load shift on a sensor-by-sensor basis.
Load shift is caused at least in part by the unequal magnetic coupling to conductors to the current transformer. The present disclosure is directed to minimizing load shift as well as addressing other problems.